This invention is related generally to plasma torches which are generally used for metal cutting, and to an improved gas-cooled electrode for such torches.
Plasma torches, also known as electric arc or plasma-arc torches, are commonly used for cutting of workpieces and operate by directing a plasma consisting of ionized gas particles toward the workpiece. In the operation of a typical plasma torch, such as illustrated in U.S. Pat. Nos. 4,324,971, 4,170,727 and 3,813,510, assigned to the same assignee as the present invention, a gas to be ionized is supplied to the front end of the torch in front of a charged electrode. The tip which is adjacent to the end of the electrode at the front end of the torch has a sufficiently high voltage applied thereto to cause a spark to jump across the gap between the electrode and tip thereby heating the gas and causing it to ionize. A pilot DC voltage between the electrode and the tip maintains a non-transferred arc known as the pilot arc. The ionized gas in the gap appears as a flame and extends outwardly from the tip. As the torch head or front end is moved towards the workpiece, a transferred or cutting arc jumps from the electrode to the workpiece since the impedance of the workpiece current path is lower than the impedance of the welding tip current path.
In conventional torches, the charged electrode is typically made of copper with a tungsten electrode insert and current flows between the tungsten insert and the torch tip or workpiece when the torch is operated. Tungsten is oxidized easily at high temperatures so that if the gas to be ionized is air, the tungsten insert becomes oxidized and is rapidly consumed, thus necessitating frequent replacement. The gas to be used for creating the plasma is typically an inert gas, such as nitrogen or argon, in order to reduce oxidation and thereby prolong electrode life. Where air is used, materials resistant to oxidation such as hafnium or zirconium have been used as the electrode insert material.
Frequently, a secondary gas flow is also provided in conventional plasma torches for various different purposes. The most common purpose of a secondary gas flow immediately adjacent and surrounding the electric arc is to cool the torch. The secondary gas helps to blow away the metal that is melted by the arc which helps to achieve a straighter kerf and therefore a cleaner cut. In conventional plasma torches, two gas lines are provided: one for supplying the plasma forming gas and the other supplying gas for the secondary gas flow. If different gases are used for the plasma forming gas and the secondary gas, operation of the torch will require two gas supplies, lines, etc. Having to use two gas lines is inconvenient to torch operators and using two gas supplies is expensive. Therefore, it is desirable to provide a plasma torch which requires only one gas line and only one gas supply. My co-pending Application Ser. No. 515,913 filed July 20, 1983, also assigned to the same assignee hereof, shows such a plasma-arc torch.
It is thus desirable to have a plasma-arc torch which uses only a single gas both for the plasma forming gas as well as the secondary gas. It is also desirable that the gas be air for reasons of availability and economy, as well as the faster speed and improved cut qulity due to the exothermic reaction of the oxygen with the iron when cutting carbon steel. It is also advantageous that the electrode be cooled so as to decrease consumption of the electrode insert.